Electromechanical transducer linear compressor and radio transmission antenna

ABSTRACT

An electromagnetic transducer which may be driven as a linear electric motor (which may be a permanent magnet motor or a variable reluctance motor) in which coils disposed around first and second cores are positioned on opposite sides of a longitudinal plane in which the longitudinal axis of the armature lies. The transducer may be arranged to work as a generator.

The present invention relates to electromechanical transducers, i.e.linear electric motors and linear electric generators. In particular,the invention relates to the electric motors which are used in linearcompressors of refrigerators or coolers, for instance those used to coolradio transmission antennae, and to generators, such as those driven byStirling engines.

The linear electric transducer of the present invention may be used as amotor in valveless compressors, particularly for driving stirling cyclecoolers or pulse tube coolers, valved compressors, for instance domesticand industrial “Freon” type refrigerators, Gifford McMahon (GM) coolersand oil-free gas compressors, pumps in clean circulation systems, suchas for medical purposes.

Linear motors previously used in these applications are not standardcomponents that are then integrated with other components. In generalthey are custom designed. Because of the wide range of applicationsrequiring linear motors there is already a range of motor designstailored to particular requirements. These include moving coil, movingmagnet and moving iron designs. Despite the range of existing motordesigns there is a lack of a design that is truly suitable forlarge-scale low cost manufacture. This invention seeks to fill this gap.

Moving coil linear electric motors of the prior art require flexiblecurrent leads and a large amount of magnet material resulting in highcost of manufacture.

Moving magnet and moving iron linear electric motors of the prior arttypically consist of a number of magnetic circuits formed by annularcores each having an air gap and an electric coil for creating amagnetic flux in the air gap. The air gas are aligned along a directionof movement of an armature received into the air gaps. The armature maybe iron or a permanent magnet or electromagnet. Such motors, though,generally suffer from a lack of robustness or complicated constructionthat is not very compatible with other aspects of linear machinetechnology.

The invention relates in particular to the geometry of the components ofthe electromechanical transducer, in particular cores for the coilswhich allow the magnetic circuits of the transducer to be closelypositioned along the direction of movement of the armature.

According to the present invention there is provided anelectromechanical transducer comprising:

-   -   a first core providing a first flux path and defining a first        air gap;    -   at least one first stationary coil disposed around a portion of        said first core; a second core providing a second flux path and        defining a second air gap;    -   at least one second stationary coil disposed around a portion of        said second core; and    -   a first armature arranged for linear motion along a longitudinal        axis through said first and second air gaps; and wherein    -   said first and second air gaps are positioned adjacent one        another in spaced apart relationship such that said longitudinal        axis passes through said first and second air gaps; and    -   said at least one first stationary coil and said at least one        second stationary coil are positioned on opposite sides of a        longitudinal plane in which said longitudinal axis lies.

There are several advantages derived from the positioning of the firstand second stationary coils on opposite sides of a longitudinal plane inwhich the longitudinal axis of the armature lies. This geometry leads tothe advantage that because the bulky first and second stationary coilscan at least partly overlap in the direction of the longitudinal axis ofthe motor, the first and second air gaps can be positioned closetogether. The positioning of the air gaps close together in thelongitudinal axis results in a more efficient use of the armature, lowermoving mass and a more compact design. The provision of coils aroundfirst and second cores means that for permanent magnet and moving irondesigns the armature does not need coils and flexible current leads arenot required. The magnetic circuits in this invention have minimalunwanted air gaps and make more efficient use of magnet material thanmany linear electric motors of the prior art. Finally, the constructionof such an electromechanical transducer is simpler than many of theelectromechanical transducers of the prior art.

The flux paths in the cores are in a single plane (i.e. planar geometry)and thus it is possible to manufacture the cores of laminations whichcan easily be stamped out of sheet metal.

Preferably the first and second cores are positioned such that the firstand second flux paths are substantially on opposite sides of thelongitudinal plane.

In this way it is possible to further reduce the size of theelectromechanical transducer, to minimise the distance between the firstand second air gaps and maximise the shape of the cores for efficiency.Furthermore, with the cores positioned in that way the size of the firstand second coils is not constrained by the need to keep the distancebetween the first and second air gaps small.

The armature may have a substantially rectangular cross-section so thatthe armature is particularly simple to construct offering lower cost ofmanufacture. If the armature is made of a permanent magnet, suchgeometry makes it easy to magnetise the armature in the correctdirection. Alternatively, the armature may comprise at least oneelectromagnet with one or more coils and soft iron cores. Flexiblecurrent leads would be required to take current into the coils.

If the at least one first stationary coil comprises two stationary coilsand the at least one second stationary coil comprises two stationarycoils, both the first stationary coils and the second stationary coilscan be arranged to give good use of space within a cylindrical housing.

Preferably the armature comprises a permanent magnet so the need for anarmature comprising coils and the required flexible current leads iseliminated.

Advantageously the electromechanical transducer is a variable reluctanceelectric motor and the armature is comprised of a soft high magneticpermeability material and thus the need for expensive permanent magneticmaterial is eliminated

The electromechanical transducer of the present invention may furthercomprise a third core for providing a third flux path and for defining athird air gap wherein the third air gap is positioned adjacent to thesecond air gap in spaced apart relationship and such that thelongitudinal axis passes through the third air gap. In such a case theat least one first stationary coil may be disposed around a portion ofthe third core for generating a magnetic field across the third air gap.

Such an arrangement can be beneficial in that the length of travel ofthe armature in the linear electric motor may be increased.

Furthermore, the electromechanical transducer may comprise a fourth corefor providing a fourth flux path and for defining a fourth air gapwherein the fourth air gap is positioned adjacent to the third air gapin spaced apart relationship and such that the longitudinal axis passesthrough said fourth air gap. In such a case the at least one secondstationary coil may be disposed around a portion of the fourth core forgenerating a magnetic field across the fourth air gap and thus thelength of travel of the armature of the electromechanical transducer maybe still further be increased.

Alternatively with additional armature components the transducer powercan be increased without increasing the transducer diameter.

Preferably the armature comprises a carriage comprising the firstarmature and a second armature separated by approximately the length ofthe second air gap in the direction of the longitudinal axis. Thisincreases the total change in flux linkage and hence increases the powerhandling capacity. This increase in capacity can be achieved withoutincreasing the motor diameter, the number of coils or the complexity ofthe control systems that provide current to the coils.

The first and second armatures may be permanent or electromagnets ormade of a soft high magnetically permeable material, the carriage maycomprise a third armature which is a magnet, the third armature beingpolarised in the opposite direction to the first and second armaturesand positioned between the first and second armatures. This furtherincreases the power handling capacity of the linear electric transducer.

Of course any number of cores may be provided all connected in the sameway as the first, second, third and fourth cores as described above.

The electromechanical transducer may be arranged to operate as a linearelectric motor wherein said at least one first and second stationarycoils are for generating magnetic fields across said first and secondair gaps respectively and said first armature is arranged for linearmotion in response to said generated magnetic fields.

The present invention also provides a linear compressor comprising sucha linear electric motor.

The present invention also provides a refrigerator comprising such alinear compressor.

The electromechanical transducer may also be arranged to operate as alinear electric generator.

The invention will now be described by way of examples only, withreference to the accompanying drawings, in which:

FIG. 1 is a schematic perspective view of a linear electric motor orlinear generator;

FIG. 2 is a plan view along axis Z of the linear electric motor of FIG.1;

FIG. 3 is a cross-section taken through the plane formed by the Y and Zaxes in FIG. 1;

FIG. 4 is a schematic diagram showing the variation in flux levels inthe cores of the linear electric motor of FIG. 1 during operation;

FIG. 5 is a schematic perspective view of a linear electric motoraccording to a second embodiment;

FIG. 6 is a cross section taken through the plane passing through axes Yand Z of the linear electric motor of FIG. 5;

FIG. 7 is a variation on the linear electric motor shown in FIG. 6;

FIG. 8 is a schematic diagram showing a typical application of a linearmotor to the drive of a compressor;

FIG. 9 is a schematic diagram showing a further typical application of alinear motor to the drive of a compressor;

FIG. 10 is a plan view along the longitudinal axis of a furtherembodiment of the linear electric motor of the present invention;

FIG. 11 is a cross-section taken through the plan A-A of FIG. 10; and

FIG. 12 is a circuit diagram of a simple drive circuit for variablereluctance embodiments of the linear motor of FIGS. 1, 5 or 10.

In the Figures, like reference numerals are used to indicate like parts.

FIG. 1 shows an electromechanical transducer which may be used either asa linear electric motor or as a linear generator. The apparatuscomprises a first core 10 which provides a first magnetic flux path 17and defines a first air gap 11. More precisely, a first pole piece 12and a second pole piece 14, both of which are part of the core 10,define between their facing surfaces the air gap 11. The facing surfacesof the preferred embodiments are substantially parallel. All elements ofthe cores 10, 20 are made of a soft high permeability magnetic material.Preferably the material should have low eddy current and hysteresislosses. Such examples are transformer iron laminations or metal powdercomposites.

In the case of laminations, the laminations are formed such that fluxentering a lamination at a pole piece 12, 14 stays mainly within thatlamination on its path around the core; in FIG. 2 the laminations are inthe plane of the paper.

The air gap 11 is dimensioned such that a first armature 50 may bepositioned between the surfaces of the pole pieces 12, 14. When thefirst armature 50 is positioned in the first air gap 11 a first magneticcircuit is completed which comprises the first core 10 (including polepieces 12, 14) and the first armature 50.

A magnetic field is generated across the first air gap 11 by passing acurrent through at least one first (electrically conducting) coil 15, 16which is coiled around a portion of the first core 10. In the embodimentillustrated in FIG. 1 two first coils 15, 16 can be provided surroundinga portion of the core 10. The two first coils 15, 16 are positionedequidistant from the first air gap 11 though this is not necessarily thecase. The coils 15, 16, when a current is passed through them, generatea magnetic field between the facing surfaces of the pole pieces 12, 14.That generated magnetic field produces a force on the first armature 50.The force will tend to move the first armature 50 into or out of the airgap 11 depending on the type of armature and its direction of magneticpolarisation.

The cross section of the core 10 is chosen to give acceptable fluxdensities for the core material. In this design this generally leads toa constant cross section in the core. The cross section may increaselocally at the pole pieces to allow lower flux densities in the armatureand the air gaps.

A second core 20, also comprising two pole pieces 22, 24, provides asecond magnetic flux path and defines a second air gap 21 just like thefirst core 10. When the first armature 50 is in the second air gap 21, asecond magnetic circuit is completed. At least one second coil 25, 26 iscoiled around the second core 20. When a current is passed through theat least one second coil 25, 26, a magnetic field is generated acrossthe second air gap 21 for driving the armature 50. In the illustratedembodiment two second coils 25, 26 are illustrated around a portion ofthe second core 20, equidistant from the pole pieces 22, 24 and thus thesecond air gap 21. When current is passed through those second electriccoils 25, 26, a magnetic field is generated across the second air gap21. That magnetic field produces a force on the first armature 50. Theforce will tend to move the first armature 50 into or out of the air gap21 depending on the type of armature and its direction of magneticpolarisation

In the embodiment illustrated in FIG. 1 the first armature 50 moves in alongitudinal axis labelled Z. This is the longitudinal axis of thelinear motor. It will be appreciated that the longitudinal axis of themotor Z passes through the centre of the armature 50 and that the firstand second air gaps 11, 21 are positioned adjacent to one another, inspaced apart relationship and such that the longitudinal axis Z passesthrough the first and second air gaps 11, 21.

In use, the first armature 50 moves along the longitudinal axis Z fromfirst air gap 11 to second air gap 21 and vice versa This is achieved byarranging for the facing surfaces of the pole pieces 12, 14, 22, 24 tolie substantially parallel to the longitudinal axis Z. Although the airgaps 11, 21 are illustrated with a rectangular shape, the air gaps canbe any two dimensional shape extending along the longitudinal axis Z ofthe motor.

As can be seen in FIG. 2, which is a plan view along the longitudinalaxis Z of the linear motor of FIG. 1, a longitudinal plane YZ,(perpendicular to the page as illustrated) in which the longitudinalaxis Z lies, provides a line of symmetry for the components forming thefirst magnetic circuit and the second magnetic circuit. Of course, thefirst magnetic circuit is positioned above the second magnetic circuitas illustrated. The first coils 15, 16 and the second coils 25, 26 arepositioned on opposite sides of the longitudinal plane YZ. This geometryallows the pole pieces 12, 14 of the first core 10 to be positionedclose to the pole pieces 22, 24 of the second core 20 as is necessaryfor them effectively to share a common armature. This is achieved whilstaccommodating the bulky coils 15, 16, 25, 26 and maintaining a practicalgeometry. The resulting apparatus for use in a cryocooler is typically80 mm in overall diameter, 35 mm long and weighs about 300 g.

The cores 10, 20 of the preferred embodiment are shaped such that therespective flux paths veer away from each other to minimise overlap. Thebest angle of divergence is about 45° from the centre line because thisangle allows most space for the coils 15, 16, 25, 26. The return pathtaking flux from one coil to the other follows a curve dictated by thenecessary cross-section and minimum overall diameter of theelectromechanical transducer.

The cores 10, 20, 30, 40 may have any shape so long as the stationarycoils 15, 16, 25, 26 may be positioned on opposite sides of alongitudinal plane in which the longitudinal axis Z lies. For example,the cores 10, 20, 30, 40 may be rectangular with an air gap in one ofthe four sides. In such a case the coils are positioned around the sidesof the rectangle, away from the air gap. Because the coils are notstacked one on top of the other, but on opposite sides of thelongitudinal plane YZ, the space inside of the cores may be effectivelyutilised to increase the size of the coils. This would not be possibleif the coils were stacked one on top of another. To utilise the space,coils 15, 16, 25, 26 may have a cross section other than rectangular.Changing the shape of the core can also lead to the ability to uselarger coils. For example, as can most clearly be seen from FIG. 2, withother shapes of core, there is enough space within the cores to addextra windings (shown dotted) in a triangular cross-section over thebasic rectangular shape. To further increase fill factor, the coils mayalso be wound with flattened wire.

The construction of the linear electric motor is kept simple because thecomponents comprising the first magnetic circuit and those comprisingthe second magnetic circuit are the same except that those components ofthe first magnetic circuit are provided on one side of the longitudinalplane YZ and those components of the second magnetic circuit aresubstantially provided on the second side of the longitudinal plane. Ineffect, the components of the second magnetic circuit are in a mirrorimage orientation to the components of the first magnetic circuit. Thus,the first and second cores 10, 20 are positioned such that the first andsecond flux paths are substantially on opposite sides of andsubstantially perpendicular to the longitudinal plane YZ. Thus, the twomagnetic circuits each have flux paths predominantly in the X Y plane.They alternate about the longitudinal plane YZ such that their fluxpaths mainly occupy different half cylinders. Of course the similarityof the components making up the flux path allows extra magnetic circuitseasily to be added (as described below) thereby increasing the power ofthe transducer or increasing the length of travel of the armature(s).

The geometry thus allows the two flux paths to be well separated exceptfor a small area around the pole pieces 12, 14, 22, 24. Although polepieces 12, 14 could actually touch pole pieces 22, 24, in practice asmall axial separation is desirable to prevent too much flux fromflowing between adjacent cores 10 and 20. Flux leaking in this wayreduces the motor force.

As can be seen from FIG. 2 and more particularly from FIG. 3 the firstarmature 50 has substantially a rectangular cross-section. FIG. 3 is across-section taken in the longitudinal plane YZ of the linear electricmotor of FIG. 1. As can be seen from FIG. 3, the facing surfaces of thenodes 12, 14, 22, 24 of the first and second cores 10, 20 aresubstantially parallel to the longitudinal plane. The air gaps 11, 21are aligned so as to form a passage along the longitudinal axis Z of themotor through which the first armature 50 can move. Therefore, it can beseen that the armature 50 is common to both magnetic circuits.

The clearance between the pole pieces 12, 14 and the first armature 50,when the first armature 50 is in the first gap 11, is an additional airgap in the magnetic circuit The smaller this gap is the less energy iswasted in driving the flux through it and the more efficient the linearelectric motor or generator will be. The same is true for the secondmagnetic circuit. With the planer geometry of the apparatus illustratedin FIG. 1, because of the simplicity, especially of the armature, thecomponents may be manufactured at low cost to high tolerance to make agenerator or engine with high efficiency.

A typical clearance between the armature and a pole piece is in theregion of 0.25 mm for a 30 W motor.

As can be seen from FIG. 3 the first armature 50 is substantially halfthe length, in the longitudinal axis, of the distance between the top ofthe first (upper) air gap 11 and the bottom of the second (lower) airgap 21.

In FIG. 3 the first armature 50 comprises a permanent magnet Thus, thelinear electric motor as illustrated is a “flux switching” machine. Themagnet is polarised in the Y direction namely in the direction from onesurface to the other surface of the pole pieces 12, 14 and is thus inthe direction across the first air gap 11. Axial movement in thelongitudinal axis Z of the first armature 50 from the first air gap 11to the second air gap 21, causes the flux of the permanent magnet of thefirst armature 50 to be switched from the first magnetic circuit of thefirst core 10 to the second magnetic circuit of the second core 20.

The simple rectangular shape of magnetic material, which is used for thefirst armature 50 in the permanent magnetic magnet version of the linearelectric motor, is easy and cheap to manufacture and to magnetise. Thearmature may be directly connected to its load and only needs simplesuspension means which are described later. Thus the first armature 50has a low moving mass and simple construction. The armature of thisembodiment may also comprise at least one electromagnet with one or morecoils and soft iron cores rather than permanent magnets. In such a case,flexible current leads are required to take current into the coils.Electromagnets are likely to be used for larger appliances. Theoperation of a motor with an electromagnet armature is the same as for apermanent magnet armature.

In operation, coils 15, 16 of the first core 10 are connected to a powersource such that current flows and the magnet of the first armature 50is repelled from core 10. At the same time coils 25, 26 of core 20 areconnected to a power source such that current flows and the magnet ofthe first armature 50 is attracted to core 20. Thus the two circuitscombine to produce a force along the longitudinal axis Z. Reversing thedirection of the currents produces a force in the opposite directionalong the longitudinal axis Z of the linear electric motor. Thus analternating current through the coils will produce an alternating forcethat can be used to power, for example, a linear compressor. Typicallythe linear electric motor will operate at about 75 Hz with a stroke of10 mm. The engine is about 80% efficient with an output shaft power ofabout 28 W.

FIGS. 3 a, b and c show three situations in which the first armature 50is positioned top dead centre, mid stroke and bottom dead centrerespectively. FIG. 4 shows, along the vertical axis, the variation influx in each magnetic circuit and in the horizontal direction, theposition of the magnet of the first armature 50 in the longitudinal axisZ of the motor. As can be seen the flux in the first magnetic circuitwhich flows around the flux path provided by the first core 10 is amaximum when the first armature 50 is in the top dead centre positionwhilst the flux in the second circuit which flows around the flux pathprovided by the second core 20 is in a minimum at this position. In thebottom dead centre position the level of flux in each magnetic circuitis reversed. As the first armature 50 passes through the mid point thelevel of flux in each magnetic circuit is equal.

The variation of flux with axial position given in FIG. 4 shows thegeneral features to be expected. The rate of flux variation tends to begreatest near the midpoint and diminishes towards top and bottom deadcentres. The actual way in which the flux varies can be controlled tosome extent by fine tuning of the geometry e.g. by shaping the polepieces 12, 14 along their axial length so as to give a small variationin the clearance between pole pieces 12, 14 and the first armature 50.

The axial force produced for a given current is proportional to the rateof change of flux and hence to the slope of this curve. Controlling therate of flux variation can be useful in controlling for instance thegeneration of unwanted harmonics.

The circuit which drives the linear electric motor when the armature 50is a permanent or electromagnet is designed in a conventional way todrive current through coils 15, 16, 25, 26 so as to drive the firstarmature 50. The coils may be electrically connected together in avariety of ways as determined by the particular embodiment.

The construction of linear electric motor illustrated in FIG. 1 may alsobe used as a variable reluctance linear electric motor in which thearmature 50 is comprised of a soft high permeable material such as iron.Preferably the material should have low eddy current losses and lowhysteresis losses e.g. laminated transformer iron.

Thus, when the linear electric motor is used as a variable reluctancemotor the armature may be a singular piece of rectangular material witha soft, high permeability. This armature is easy and cheap tomanufacture and no magnets are necessary. Such motors can be designed towork in hostile environments, for example, at high temperature.

In the variable reluctance machine the movement of the iron armature 50varies the reluctance of the two magnetic circuits associated with thefirst core 10 and the second core 20. With the first armature 50 in themid position both magnetic circuits have nominally the same reluctance.As the moving member leaves the second air gap 21 and engages more withthe first magnetic circuit by entering the first air gap 11, thereluctance of the second magnetic circuit associated with the secondcore 20 increases and the reluctance of the first magnetic circuitassociated with the first core decreases. Reversing the direction ofmovement reverses reluctance changes in the two circuits.

If the first coils 15, 16 are energised whilst the second coils 25, 26remain un-energised, a force will be exerted on the soft highpermeability first armature 50 in the direction towards the first airgap 11 such that the reluctance of the first magnetic circuit decreases.This force is due to the dependence of gap energy on air gap volume—asthe reluctance decreases the gap energy also decreases and work is doneon the first armature 50.

If the second magnetic circuit is energised by passing a current throughsecond coils 25, 26 and the coils 15, 16 are switched off, then theforce is reversed.

It will thus be seen that if the first and second magnetic circuits arealternately switched on and off then an alternating force is generatedon the first armature 50 that can be used to drive a linear compressor.FIG. 12 shows a simple circuit which can achieve this. An alternatingcurrent is passed through first coils 15, 16 which are placed inparallel with second coils 25, 26. Coils 15, 16 are placed in serieswith a first diode 18 whilst second coils 25, 26 are placed in serieswith a second diode 28. The directions of the first diode 18 and seconddiode 28 are opposite. Thus, if an alternating voltage is applied, thediodes 18, 28 conduct for alternative half cycles producing analternating force in the motor. It will be appreciated that there aremany different approaches to producing the appropriate switching in thecoils 15, 16, 25, 26, which are known in the field of reluctance motors.

If the first armature 50 is comprised of a soft, high permeabilitymagnetic material and the cores 10, 20 and coils 15, 16, 25, 26 arearranged in the way illustrated in FIGS. 1-3 then the apparatus may beused as a variable reluctance generator if an appropriate electroniccircuit is synchronised with the movement of moving member. If thearmature is made of a permanent magnetic material or electromagnetpolarised in the direction as illustrated in FIG. 3 and as describedabove then movement of the magnet will change the flux through eachmagnetic circuit and hence the varying flux linkage will generateinduced voltages in the coils.

It will be appreciated that several sets of cores 10, 20 and associatedcoils 15, 16, 25, 26 may be placed in series one after another either toincrease the armature force or to increase the travel of the armature50.

FIG. 5, shows a embodiment in which the force exerted by the armature 50is doubled whilst the number of coils 115, 116, 125, 126 remains thesame. In this embodiment a third core 30 is positioned underneath (asillustrated) and in the same orientation as the first core 10 and afourth core 40 is positioned underneath and in the same orientation asthe second core 20. The third core 30 and fourth core 40 have the samegeometry as the first and second cores 10, 20 and have associated airgaps 31, 41. The cores 10, 20, 30, 40 are positioned such that the firstair gap 11 is adjacent to the second air gap 21, that the second air gap21 is adjacent to the third air gap 31 and that the third air gap 31 isadjacent to the fourth air gap 41.

As can be seen from FIG. 5 only four coils are used and this is achievedby providing the first coils 115, 116 around both the first core 10 andthe third core 30. Thus, when a magnetic field is generated across thefirst air gap 11 a magnetic field is also generated across the third airgap 31. A similar arrangement is used in relation to second core 20 andfourth core 40 in which the second coils 125, 126 are shared. Thus, whenthe second coils 125, 126 are energised a magnetic field will begenerated across the second air gap 21 as well as across the fourth airgap 41. Of course, only two coils are necessary for the functioning ofthe motor, but any number of coils may be used.

The decision about whether each core should have its own associated coilor to share coils as in FIG. 5 is likely to be determined byconsideration of manufacturing cost

FIG. 6 illustrates the air gaps in the linear motor of FIG. 5. As can beseen, a carriage 150 is comprised of first and second armatures 51, 52which are magnets (permanent or electromagnet) polarised in the samedirection attached together but separated such that the movement ofmagnet 51 in air gaps 11 and 21 corresponds with the movement of magnet52 in air gaps 31 and 41. The magnets are connected together by anon-magnetic member 55 comprised of a material such as plastic.

As can be seen, in the top dead centre position the first armature 51 isfully in the first air gap 11 and the second armature 52 is fully in thethird air gap 31. Likewise in the bottom dead centre position thearmatures 51 and 52 are fully in the second and fourth air gapsrespectively. Currents through coils 115, 116, 125, 126 in theappropriate direction cause the armatures 51, 52 to be repelled from airgaps 11, 31 and attracted to air gaps 21, 41 to produce an axial force,the force being the combined forces acting on armatures 51, 52. Theforce is reversed if the currents are reversed and the general operationis similar to that already described for the basic two core unit of FIG.1.

It will be understood that the first and second coils 115, 116, 125, 126could be each positioned around more than two cores or that acombination of separate coils around individual cores and coils aroundmultiple cores could be included in the same linear electric motor.

The embodiment illustrated in FIGS. 5 and 6 are equally applicable tovariable reluctance motors as described above in the same way as for theapparatus of FIG. 1 in which use the first and second armatures 51, 52are made of a soft high permability material. The apparatus of FIG. 5may be used as a generator with either permanent or electromagnets inthe armatures 51, 52 or with armatures 51, 52 made of a soft highpermeability magnetic material.

The embodiment illustrated in FIG. 7 is a variation on the embodimentillustrated in FIG. 6. Embodiment illustrated in FIG. 7 only works withflux switching machines in which the armatures are comprised ofpermanent or electro-magnets. In this embodiment an additional thirdmagnet 53 is incorporated between the first magnet 51 and the secondmagnet 52 which have the same separation as in the embodimentillustrated in FIG. 6. In the FIG. 7 embodiment the third magnet 53 ispolarised in the opposite direction to magnets 51 and 52 such that thechange of flux linkage as the armature 50 moves from top dead centre tobottom dead centre is increased for the second and third magneticcircuits associated with the second and third cores 20, 30 respectively.In this way the power handling capacity is increased still further.

It is clear that further units can be added and integrated in a similarway and that the apparatus may also be used as a generator.

FIG. 8 shows a linear motor such as the one illustrated in FIG. 5attached to a linear compressor comprising a piston 80 in a cylinder 85.The piston 80 is connected to the armature via an attachment means 57.The piston 80, armature and attachment means 57 are mounted on two setsof suspension springs 70. The suspension springs 70 constrain the motionof the piston/armature assembly to an accurate linear motion along themotor/compressor axis. The radial clearance between the piston andcylinder is kept very small so that fluid leakage is acceptably low.This type of seal does not have contacting surfaces and is called aclearance seal. The elimination of any wearing surfaces in this type ofmachine allows it to achieve long life with oil free operation.

In the embodiment illustrated in FIG. 8, the motor is attached to an endof a self contained compressor assembly whereas in the embodiment ofFIG. 9, the armature 50 is mounted between suspension springs 72, 74 toform an integrated compressor and motor. For the embodiment of FIG. 9,the armature also serves as a structural component that connects to twosets of springs together. For this purpose it will be designed to havethe required stiffness and strength. These compressors might be valvedor valveless compressors, stirling cycle coolers or pulse tube coolers.

Examples of valve compressors which may be driven by the linear electricmotors described above are domestic and industrial “Freon” typerefrigerators, GM (Gifford McMahon) coolers and oil-free gascompressors. The linear electric motors may also drive pumps, forexample, clean circulation system pumps for medical purposes.

In generator mode the apparatus described above may be used in stirlingengines.

A particular application of the coolers or refrigerators driven by theselinear electric motors are for the cooling of radio communicationsantennae.

In the embodiments of FIGS. 1 to 9 the coils are wound on separateformers. Therefore, in order to allow those wound coils to be fitted tothe cores, the cores themselves are made of three components so that thecoils can be fitted before the cores are assembled. A different designprinciple in which the number of components is minimised and the amountof materials used is minimised results in a slightly differentconstruction which is illustrated in FIGS. 10 and 11. In this furtherembodiment the cores 210, 220 are one-piece cores which can be eitheriron laminations (parallel to the plane of the paper) or made from asoft magnetic composite material.

The coils 215, 216, 225, 226 are then wound around the coils in the sameway as the coils of a toroidal transformer are wound. In this way thecores 210, 220 do not need to be split apart in order to fit pre-woundcoils. Thus, the motor is of a very simple construction and has fewcomponents which, from a manufacturing point of view, is highlyattractive.

The one-piece cores of FIG. 10 made of a soft magnetic composite havevery low bulk electrical conductivity compared to soft iron laminations.Thus, there is no need to laminate such cores to avoid any current andvarying three-dimensional magnetic fields can be used with only minimaleddy current losses thereby allowing more flexibility in design.

With the use of soft magnetic composites the eddy current losses arereduced so that the transducer can be used at higher frequency. Thus,the possibility of the eddy current loss associated with fieldcomponents in the vicinity of the pole pieces of the embodiments ofFIGS. 1 to 7 where the flux spreads itself across the coils (due to theplanar field structure) when the core is laminated, is reduced. Thoselosses, which are acceptable for low frequency applications, can beminimised such that operation at high frequencies is possible.

Of course a number of coils of the FIG. 10 embodiment could be stackedone on top of another in the same way as illustrated in FIG. 6. However,in such a case, it is likely that each core would have its own separatetoroidal coil.

Further refinements of the core are also illustrated in FIGS. 10 and 11.Lugs 201 are incorporated in the cores which is one way in which thecomponents could be mounted in a motor. In FIG. 11 it can be seen thatthe pole pieces 212, 214, 222, 224 have been shaped along thelongitudinal axis Z of the motor so as to reduce the leakage between themagnetic circuits whilst retaining the proximity of the actual polepieces. This is achieved by the greater separation of the cores whichresults because the pole pieces have a greater dimension in the Zdirection than the remainder of the core. Thus, although the pole piecesof different cores are still positioned close together, the remainingpart of the cores are positioned further apart thereby achieving lowerflux leakage.

1. An electromechanical transducer comprising: a first core providing a first flux path and defining a first air gap; at least one first stationary coil disposed around a portion of said first core; a second core providing a second flux path and defining a second air gap; at least one second stationary coil disposed around a portion of said second core; and a first armature arranged for linear motion along a longitudinal axis through said first and second air gaps; and wherein said first and second air gaps are positioned adjacent one another in spaced apart relationship such that said longitudinal axis passes through said first and second air gaps; and said at least one first stationary coil and said at least one second stationary coil are positioned on opposite sides of a longitudinal plane in which said longitudinal axis lies.
 2. An electromechanical transducer motor according to claim 1, wherein said first and second air gaps are formed between two opposed surfaces of pole pieces of said respective first and second cores.
 3. An electromechanical transducer according to claim 2, wherein said opposed surfaces are substantially transverse to said longitudinal plane.
 4. An electromechanical transducer according to claim 1, wherein said first and second cores are positioned such that said first and second flux paths are substantially on opposite sides of said longitudinal plane.
 5. An electromechanical transducer according to claim 1, wherein said first and second cores have substantially the same geometry.
 6. An electromechanical transducer according to claim 5, wherein said first and second cores are positioned substantially symmetrically to each other with respect to said longitudinal plane.
 7. An electromechanical transducer according to claim 1, wherein said first armature has a substantially rectangular cross-section.
 8. An electromechanical transducer according to claim 1, wherein said at least one first stationary coil is wound in a toroidal fashion around said first core and said at least one second stationary coil is wound in a toroidal fashion around said second core.
 9. An electromechanical transducer according to claim 1, wherein said at least one first stationary coil comprises two stationary coils and said at least one second coil stationary comprises two stationary coils.
 10. An electromechanical transducer according to claim 1, wherein said first and second flux paths are generally perpendicular to said longitudinal axis.
 11. An electromechanical transducer according to claim 1, wherein said first armature has a length in the direction of said longitudinal axis substantially equal to the length of said air gaps in the direction of said longitudinal axis.
 12. An electromechanical transducer according to claim 1, wherein said first and second cores are made from a soft magnetic composite.
 13. An electromechanical transducer according to claim 1, wherein said cores have a larger dimension in the direction of said longitudinal axis in proximity to said air gaps than elsewhere.
 14. An electromechanical transducer according to claim 1, further comprising: a third core for providing a third flux path and for defining a third air gap, wherein said third air gap is positioned adjacent to said second air gap in spaced apart relationship and such that said longitudinal axis passes through said third air gap.
 15. An electromechanical transducer according to claim 14, wherein said at least one first stationary coil is disposed around a portion of said third core for generating a magnetic field across said third air gap.
 16. An electromechanical transducer according to claim 14, further comprising a fourth core for providing a fourth flux path and for defining a fourth air gap; wherein said fourth air gap is positioned adjacent to said third air gap in spaced apart relationship and such that said longitudinal axis passes through said fourth air gap.
 17. An electromechanical transducer according to claim 16, wherein said at least one second stationary coil is disposed around a portion of said fourth core for generating a magnetic field across said fourth air gap.
 18. An electromechanical transducer according to claim 14, further comprising a carriage arranged for linear motion along said longitudinal axis through said first and second air gaps and which comprises said first armature and a second armature, said armatures being separated by approximately the length of said second air gap in the direction of said longitudinal axis.
 19. An electromechanical transducer according to claim 18, wherein said first and second armatures are magnets both polarised in a direction substantially perpendicular to said longitudinal axis.
 20. An electromechanical transducer according to claim 19, wherein said carriage comprises a third armature which is a magnet, said third magnet being polarised in the opposite direction to said first and second armatures and positioned between said first and second armatures.
 21. An electromechanical transducer according to claim 1, wherein said first armature comprises a magnet.
 22. An electromechanical transducer according to claim 21, wherein said magnet is a permanent magnet which is polarised in a direction substantially perpendicular to said longitudinal axis.
 23. An electromechanical transducer according to claim 21, wherein said magnet comprises at least one electromagnet with one or more coils and soft iron cores polarisable in a direction substantially perpendicular to said longitudinal axis.
 24. An electromechanical transducer according to claim 18, wherein said first and second armatures are comprised of a soft high magnetically permeable material.
 25. An electromechanical transducer according to claim 1, wherein said first armature is comprised of a soft high magnetically permeable material.
 26. An electromechanical transducer according to claim 1 arranged to operate as a linear electric motor, wherein said at least one first and second stationary coils are for generating magnetic fields across said first and second air gaps respectively and said first armature is arranged for linear motion in response to said generated magnetic fields.
 27. An electromechanical transducer according to claim 26, further comprising an electric circuit for generating current in said at least one first stationary coil and said at least one second stationary coil.
 28. An electromechanical transducer according to claim 1 arranged to operate as a linear electric generator.
 29. A linear compressor comprising a linear electric motor according to claim
 26. 30. A linear compressor according to claim 29, further comprising a piston connected at an end of said first armature.
 31. A linear compressor according to claim 29, further comprising suspension springs for suspending said first armature in said air gaps.
 32. A refrigerator comprising a linear compressor of claim
 29. 33. A radio communications antenna comprising a refrigerator of claim
 32. 34. (Canceled) 